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7. LIMITATIONS
The minimum retroreflectivity levels presented in this study are the product of computer models and simplifying assumptions based on numerous other research efforts. Therefore, these recommendations are subject to the following limitations:
- The TARVIP analysis used a preview time of 2.2 seconds. It was assumed that this was an adequate preview time for drivers to safely and comfortably navigate their vehicles. Drivers requiring a higher preview time will need pavement markings with higher RL values than those recommended herein.
- The TARVIP analysis considered only straight roadways and longitudinal pavement markings. Curved roadway segments, especially those with small low radii, place the approaching pavement markings in a different location in the projected headlamp beam pattern than they would be on a straight roadway. Consequently, pavement markings in a curved roadway segment may be more or less visible to drivers than those on a straight segment.
- Only dry, clear weather conditions were considered. Standing water, rain, snow, and fog can all have a significant negative impact upon pavement marking visibility and require much higher retroreflectivity levels than dry and clear conditions to achieve the same visibility distance.
- This analysis assumes that vehicle headlamps are in good working condition and windshields are clean.
- The only pavement marking material that was modeled was white alkyd paint with beads. Although TARVIP provides the capability to model other materials, there is no available information regarding crucial details such as bead quality and size or the marking material thickness associated with the retroreflectivity matrices used in the TARVIP models of alternative materials. As a result, alternative pavement marking materials that cannot be adequately modeled with existing information may exhibit different visibility performance s with respect to distance, when compared to white alkyd paint with beads.
- The TARVIP analysis used pavement surface retroreflectivity matrices developed by researchers at the University of Ohio based on readings taken from specific surfaces. It was assumed that these matrices are representative of road surfaces in use throughout the United States.
- It was assumed that the 50 th percentile UMTRI-2004 headlamp is representative of the headlamps in the U.S. vehicle fleet. There may be a significant number of vehicles operating in the United States using headlamps with inferior performance to that of the UMTRI-2004 headlamp.
- The RRPM analysis assumes that the deployed RRPMs on a roadway provide long detection distances, and thus, long preview times. However, RRPM visibility can deteriorate quickly from the “new” condition. Therefore, it was assumed herein that the RRPMs are in adequate working order when applying the discount factor of 45 percent to the pavement marking minimum retroreflectivity levels.
- Drivers older than 62 years old may require greater pavement marking retroreflectivity than the minimum levels presented here.
- The retroreflectivity levels were developed without consideration of driver needs when encountering glare from oncoming vehicles.
The retroreflectivity values recommended by this research should be considered minimum maintenance levels, and markings should be replaced or scheduled for replacement before falling below these recommended levels. The retroreflectivity values recommended by this research are not intended to account for worst-case conditions or all possible combinations of roadway geometry, speed, and pavement marking configurations. In summary, engineering judgment should be used to evaluate whether or not conditions warrant the use of pavement markings with higher maintained retroreflectivity.
It should also be noted that winter maintenance activities can severely damage the retroreflective performance of pavement markings. In addition, nonconcentric driving behavior around horizontal curves can also wear markings faster along the length of the curves than on adjacent tangent sections. The recommended minimum retroreflectivity levels presented in this report should be considered as applicable to sections of the marking representative of the marking over the length of the roadway and not to specific points along the roadway.
